Re: [PATCH v2] mm: compaction: skip memory hole rapidly when isolating migratable pages

[Mel Gorman]

On Tue, Jun 13, 2023 at 04:55:04PM +0800, Baolin Wang wrote:
> On some machines, the normal zone can have a large memory hole like
> below memory layout, and we can see the range from 0x100000000 to
> 0x1800000000 is a hole. So when isolating some migratable pages, the
> scanner can meet the hole and it will take more time to skip the large
> hole. From my measurement, I can see the isolation scanner will take
> 80us ~ 100us to skip the large hole [0x100000000 - 0x1800000000].
> 
> So adding a new helper to fast search next online memory section
> to skip the large hole can help to find next suitable pageblock
> efficiently. With this patch, I can see the large hole scanning only
> takes < 1us.
> 
> [    0.000000] Zone ranges:
> [    0.000000]   DMA      [mem 0x0000000040000000-0x00000000ffffffff]
> [    0.000000]   DMA32    empty
> [    0.000000]   Normal   [mem 0x0000000100000000-0x0000001fa7ffffff]
> [    0.000000] Movable zone start for each node
> [    0.000000] Early memory node ranges
> [    0.000000]   node   0: [mem 0x0000000040000000-0x0000000fffffffff]
> [    0.000000]   node   0: [mem 0x0000001800000000-0x0000001fa3c7ffff]
> [    0.000000]   node   0: [mem 0x0000001fa3c80000-0x0000001fa3ffffff]
> [    0.000000]   node   0: [mem 0x0000001fa4000000-0x0000001fa402ffff]
> [    0.000000]   node   0: [mem 0x0000001fa4030000-0x0000001fa40effff]
> [    0.000000]   node   0: [mem 0x0000001fa40f0000-0x0000001fa73cffff]
> [    0.000000]   node   0: [mem 0x0000001fa73d0000-0x0000001fa745ffff]
> [    0.000000]   node   0: [mem 0x0000001fa7460000-0x0000001fa746ffff]
> [    0.000000]   node   0: [mem 0x0000001fa7470000-0x0000001fa758ffff]
> [    0.000000]   node   0: [mem 0x0000001fa7590000-0x0000001fa7ffffff]
> 
> Signed-off-by: Baolin Wang <baolin.wang@xxxxxxxxxxxxxxxxx>

This may only be necessary for non-contiguous zones so a check for
zone_contiguous could be made but I suspect the saving, if any, would be
marginal.

However, it's subtle that block_end_pfn can end up in an arbirary location
past the end of the zone or past cc->free_pfn. As the "continue" will update
cc->migrate_pfn, that might lead to errors in the future. It would be a
lot safer to pass in cc->free_pfn and do two things with the value. First,
there is no point scanning for a valid online section past cc->free_pfn so
terminating after cc->free_pfn may save some cycles. Second, cc->migrate_pfn
does not end up with an arbitrary value which is a more defensive approach
to any future programming errors.

-- 
Mel Gorman
SUSE Labs